what happen if someone in 5d use harpy feather duster or MST targeted on Speed World ?
The one who has the most money wins.
Victory is assured.
Sorry, but only speed cards can be used when Speed World is on the field.
Actually, you can, but you'll pay a hefty price for it.
>If a player activates a Spell Card other than a "Speed Spell" Spell Card, that player take 2000 damage. During each player's Standby Phase, put 1 Speed Counter on this card for the turn player (max. 12 for each player). When a monster inflicts damage, remove Speed Counters of the attacked Player from this card for each multiple of 1000 damage taken.